Structural and functional characterization of human Iba proteins

Area of Science:

• Cell Biology
• Structural Biology
• Biochemistry

Background:

• Ionized calcium-binding adapter molecule 1 (Iba1) is a 17-kDa protein involved in actin binding and cross-linking, crucial for cellular functions like phagocytosis.
• Iba2 is a homolog of Iba1, suggesting potential shared functions in cytoskeletal dynamics and cellular responses.

Purpose of the Study:

• To elucidate the crystal structure and biophysical properties of human Iba2.
• To compare the F-actin binding and cross-linking activities of Iba2 with Iba1.
• To investigate the cellular localization and recruitment dynamics of Iba2 and Iba1 during bacterial invasion.

Main Methods:

• X-ray crystallography was used to determine the structure of human Iba2.
• Analytical ultracentrifugation and equilibrium dialysis were employed to study Iba2 dimerization and calcium binding.
• Sedimentation experiments, microscopy, and fluorescent protein expression in HeLa cells were used to assess F-actin interactions and cellular localization.
• A Shigella invasion model was utilized to examine Iba protein recruitment during bacterial entry.

Main Results:

• The crystal structure of Iba2 revealed EF-hand motifs incapable of significant Ca2+ binding.
• Iba2 forms homodimers stabilized by disulfide bridges and zinc ions, with Ca2+-independent dimerization under reducing conditions.
• Both Iba1 and Iba2 demonstrated potent F-actin binding and cross-linking activities, inducing F-actin bundles.
• While both proteins localized with F-actin in cells, Iba1 showed slightly higher recruitment to cellular projections and the bacterial invasion zone during Shigella infection.

Conclusions:

• Iba2 possesses significant F-actin binding and cross-linking capabilities, comparable to Iba1, despite lacking functional calcium-binding EF-hand motifs.
• Dimerization of Iba2 is primarily mediated by disulfide bonds and zinc ions, independent of calcium.
• Both Iba1 and Iba2 are recruited to sites of bacterial invasion, highlighting their roles in host-pathogen interactions and cytoskeletal rearrangements.